I'm honestly confused aout the usage of the words "pure" and "corrupted" here..
I understand that some people don't like the sister in the relationship, but I don't think that
makes her generic of slutty. I'm pretty sure she still wants do be daddies number one.
But, even if we would cut the sister out, she would still be a corrupted girl, simply
because there is nothing pure in incest. Incest is, at least according to nearly all moral systems known to me,
inherently corrupt. Her being willing to engage in incest with her father means that she is willing to
leave the confines of morality which is not pure but corrupt.
I havn't played the game in some time, but I think that the fact that incest is not "normal" is actualy adressed, and taboos mean that their moral system prohibits such acts.
The corruption thing is a tag and a concept that gets applied to a lot of these games, stories, fantasies, and so on. It's the idea that you need to change the girl in order for her to sleep with you, or that you want to do that. At least that's my perception of it. It is definitely an approach in the fictional world, as well as real life, where guys might tear away at a woman's self-confidence, get her drunk, or play mind games with her in order to have sex with her. In a game, that's fine.
The pureness has two meanings for someone like me. First, it means that they remain like how you first met them in the game. If the girl is a shy, nerd at the beginning of the game, and you turn her into a very social party girl along the way to seducing her, then she's been changed and is no longer pure to her original concept. But there's the literal meaning as well. When we first meet Elly and Ada, both are "pure" in the regular meaning of the word. But as in so many games, that pureness is viewed as an obstacle or something that must be worn away, without any recognition that the pureness was a key character trait. Some games try to compensate by having the girl gain stupidity as she loses "purity", but that never goes well in my mind. The smart, pure girl slowly turns into a progressively corrupted idiot that bears only a physical resemblance to her earlier self.